/*
Copyright 2014 The Kubernetes Authors All rights reserved.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package cache

import (
	"github.com/GoogleCloudPlatform/kubernetes/pkg/util"
	"github.com/golang/glog"
	"time"
)

// ExpirationCache implements the store interface
//	1. All entries are automatically time stamped on insert
//		a. The key is computed based off the original item/keyFunc
//		b. The value inserted under that key is the timestamped item
//	2. Expiration happens lazily on read based on the expiration policy
//	3. Time-stamps are stripped off unexpired entries before return
type ExpirationCache struct {
	cacheStorage     ThreadSafeStore
	keyFunc          KeyFunc
	clock            util.Clock
	expirationPolicy ExpirationPolicy
}

// ExpirationPolicy dictates when an object expires. Currently only abstracted out
// so unittests don't rely on the system clock.
type ExpirationPolicy interface {
	IsExpired(obj *timestampedEntry) bool
}

// TTLPolicy implements a ttl based ExpirationPolicy.
type TTLPolicy struct {
	//	 >0: Expire entries with an age > ttl
	//	<=0: Don't expire any entry
	Ttl time.Duration

	// Clock used to calculate ttl expiration
	Clock util.Clock
}

// IsExpired returns true if the given object is older than the ttl, or it can't
// determine its age.
func (p *TTLPolicy) IsExpired(obj *timestampedEntry) bool {
	return p.Ttl > 0 && p.Clock.Since(obj.timestamp) > p.Ttl
}

// timestampedEntry is the only type allowed in a ExpirationCache.
type timestampedEntry struct {
	obj       interface{}
	timestamp time.Time
}

// getTimestampedEntry returnes the timestampedEntry stored under the given key.
func (c *ExpirationCache) getTimestampedEntry(key string) (*timestampedEntry, bool) {
	item, _ := c.cacheStorage.Get(key)
	// TODO: Check the cast instead
	if tsEntry, ok := item.(*timestampedEntry); ok {
		return tsEntry, true
	}
	return nil, false
}

// getOrExpire retrieves the object from the timestampedEntry iff it hasn't
// already expired. It kicks-off a go routine to delete expired objects from
// the store and sets exists=false.
func (c *ExpirationCache) getOrExpire(key string) (interface{}, bool) {
	timestampedItem, exists := c.getTimestampedEntry(key)
	if !exists {
		return nil, false
	}
	if c.expirationPolicy.IsExpired(timestampedItem) {
		glog.V(4).Infof("Entry %v: %+v has expired", key, timestampedItem.obj)
		// Since expiration happens lazily on read, don't hold up
		// the reader trying to acquire a write lock for the delete.
		// The next reader will retry the delete even if this one
		// fails; as long as we only return un-expired entries a
		// reader doesn't need to wait for the result of the delete.
		go func() {
			defer util.HandleCrash()
			c.cacheStorage.Delete(key)
		}()
		return nil, false
	}
	return timestampedItem.obj, true
}

// GetByKey returns the item stored under the key, or sets exists=false.
func (c *ExpirationCache) GetByKey(key string) (interface{}, bool, error) {
	obj, exists := c.getOrExpire(key)
	return obj, exists, nil
}

// Get returns unexpired items. It purges the cache of expired items in the
// process.
func (c *ExpirationCache) Get(obj interface{}) (interface{}, bool, error) {
	key, err := c.keyFunc(obj)
	if err != nil {
		return nil, false, KeyError{obj, err}
	}
	obj, exists := c.getOrExpire(key)
	return obj, exists, nil
}

// List retrieves a list of unexpired items. It purges the cache of expired
// items in the process.
func (c *ExpirationCache) List() []interface{} {
	items := c.cacheStorage.List()

	list := make([]interface{}, 0, len(items))
	for _, item := range items {
		obj := item.(*timestampedEntry).obj
		if key, err := c.keyFunc(obj); err != nil {
			list = append(list, obj)
		} else if obj, exists := c.getOrExpire(key); exists {
			list = append(list, obj)
		}
	}
	return list
}

// ListKeys returns a list of all keys in the expiration cache.
func (c *ExpirationCache) ListKeys() []string {
	return c.cacheStorage.ListKeys()
}

// Add timestamps an item and inserts it into the cache, overwriting entries
// that might exist under the same key.
func (c *ExpirationCache) Add(obj interface{}) error {
	key, err := c.keyFunc(obj)
	if err != nil {
		return KeyError{obj, err}
	}
	c.cacheStorage.Add(key, &timestampedEntry{obj, c.clock.Now()})
	return nil
}

// Update has not been implemented yet for lack of a use case, so this method
// simply calls `Add`. This effectively refreshes the timestamp.
func (c *ExpirationCache) Update(obj interface{}) error {
	return c.Add(obj)
}

// Delete removes an item from the cache.
func (c *ExpirationCache) Delete(obj interface{}) error {
	key, err := c.keyFunc(obj)
	if err != nil {
		return KeyError{obj, err}
	}
	c.cacheStorage.Delete(key)
	return nil
}

// Replace will convert all items in the given list to TimestampedEntries
// before attempting the replace operation. The replace operation will
// delete the contents of the ExpirationCache `c`.
func (c *ExpirationCache) Replace(list []interface{}) error {
	items := map[string]interface{}{}
	ts := c.clock.Now()
	for _, item := range list {
		key, err := c.keyFunc(item)
		if err != nil {
			return KeyError{item, err}
		}
		items[key] = &timestampedEntry{item, ts}
	}
	c.cacheStorage.Replace(items)
	return nil
}

// NewTTLStore creates and returns a ExpirationCache with a TTLPolicy
func NewTTLStore(keyFunc KeyFunc, ttl time.Duration) Store {
	return &ExpirationCache{
		cacheStorage:     NewThreadSafeStore(Indexers{}, Indices{}),
		keyFunc:          keyFunc,
		clock:            util.RealClock{},
		expirationPolicy: &TTLPolicy{ttl, util.RealClock{}},
	}
}